In the field of medicine phantoms or simulators have significant utility. Phantoms allow for the evaluation, analysis, and performance optimization of various imaging devices including magnetic resonance imaging (MRI) devices. They are more readily available and provide better consistency than use of a living specimen or cadaver.
In some biological tissues the diffusion of water is dependent on its interactions with the surrounding environment. Macromolecular structures, fibers, membranes and the dimension of the volumes containing the diffusing water reveal different information regarding the tissue in which the diffusion occurs. This information may provide insight into the architecture of anatomical and sub-anatomical structures; for example axonal fibers in the central nervous system.
Diffusion tensor imaging (DTI) is an MRI based imaging method that measures the anisotropic rate of water diffusion. The high degree of organization of white matter in the brain leads water to diffuse more rapidly in directions along white matter tracts because physical barriers such as mylinated axonal walls restrict water movement in other directions.
This unique ability of DTI to closely examine the fine structural changes of biological tissue by measuring anisotropic diffusion of water is very useful in determining the fine structure of white matter, delineating the boundaries of necrotic or damaged tissue, detection and confirmation of neurodegenerative diseases and brain disorders not found out by general medical imagining. The importance of DTI is underscored by the fact that it is the only way of studying white matter structure in vivo. This is a key element in being able to understand how these connections in the brain are affected during the progression of various diseases, and how cognitive and behavioral systems are linked to these changes.
The ability of DTI to describe connectivity in the brain has been clinically relevant for the study of neurological disorders as it can reveal abnormalities in white matter fiber structure and provide models of brain functionality.
This connectivity relies on the fact that functioning white matter consists of multiple axons contained within myelin sheaths with multiple axons arranged collinearly to form fascicles or bundles of nerve cells. The extra-cellular water contained in the spaces between myelin sheaths experiences anisotropic (directionally dependent) diffusion along the direction of the fascicle as the space is heavily restricted in perpendicular directions.
Presently, MR diffusion protocols may be used to measure both the rate and anisotropic property of diffusion. Differences in the rate of diffusion are associated with differences in cellular density which may vary between adjacent sub-anatomical structures—for example between ventricles and white matter, or, between healthy and some diseased tissues. For example cystic tumours would be characterized by regions of low (hypo-) cellularity and dense tumours such as a fibrous metastatic tumour would be characterized by high (hyper-) cellularity. The directionality (anisotropy) of diffusion is associated with tissue organization. Greater diffusional anisotropy indicates a more strongly directed diffusion, or highly structured tissue, such as white matter fiber bundles.
The practical applicability of DTI is limited by variation in the diffusion indices in different MR scanners; its inability to resolve multiple fiber populations; as well as by variations caused by the use of different imaging parameters (e.g. those used in longitudinal or multicenter trials). Therefore, the development of a standard DTI phantom to serve as a baseline for calibrated measurement and validated imaging would find utility.
Presently diffusion phantoms typically focus on either mimicking the rate or anisotropic nature of diffusion but not both simultaneously.